1. Field of the Invention
The present invention relates to an electrically insulating method for a car power inverter comprising semiconductor switching elements and to a reduction in the size of the inverter and the improvement of reliability of the inverter.
2. Description of the Prior Art
FIG. 4 is a circuit block diagram showing the configuration of the power source of a prior art car power inverter. The figure shows only a one-phase circuit for a switching portion for driving a three-phase AC load. Circuits for the other two phases are omitted because they are identical to the above circuit (refer to the whole circuit block diagram of FIG. 5).
In FIG. 4, reference numeral 1 denotes a high-voltage DC power source for driving an unshown AC load. For such application as electric cars, several tens to several hundreds of volts is generally used. The above DC power source 1 is electrically insulated from a car body for safety. Reference numeral 2 signifies two switching elements forming one arm of a switching portion. The two switching elements 2 are arranged in series between both ends of the DC power source 1. A power element typified by an IGBT (Insulated Gate Bipolar Transistor) is generally used as the switching element 2, and DC power from the above DC power source 1 is inverted into 3-phase AC power by the switching operation of the switching elements 2 forming 3 arms connected in parallel as will be described hereinafter. Denoted by 3 is a fly wheel diode, paired with the above switching element 2, for returning a reflux current at the time of switching to the AC load or the DC power source 1.
Denoted by 4 is a circuit for driving the above switching element 2 and protecting it from overheating, short-circuit current and the like. This circuit is connected to the drive signal output port and switching abnormal signal input port of a microcomputer 5 through photocouplers 8. Reference numeral 10 denotes a capacitor for smoothing a ripple current at the time of switching. The drive voltage of the microcomputer 5 is supplied from a low-voltage DC power source 6 and regulated to 5 V by a general constant-voltage regulator circuit 7.
Thus, in the case of an electric car or hybrid electric car, two different DC power sources are generally used. That is, a 12 V battery generally used for automobiles is used as the low-voltage DC power source 6 and the GND potential of the power source is grounded to the car body so that it is made equal to the potential of the car body.
The high-voltage DC power source 1 is electrically insulated from the car body in most cases because it generates a high voltage and the drive and protective circuits 4 are electrically insulated from the microcomputer 5 using insulating elements for the prevention of a safety problem such as an electric shock and the operation principle of the switching elements (difference in operation reference voltage). The photocoupler 8 which is relatively inexpensive and easily acquired is used as the insulating element.
The body earth is represented by "Z" below.
The whole circuit block diagram of the car power inverter of FIG. 5 will be described hereinunder. Elements having the same reference symbols as in FIG. 4 have the same functions.
A high-voltage power inverter 9 which is the main circuit of a car power inverter comprises a DC power source 1, a smoothing capacitor 10 and a switching portion consisting of 3 pairs of switching elements 2 and 3 pairs of fly wheel diodes 3 connected in parallel, and inverts a DC voltage obtained by smoothing a ripple current at the time of switching between the terminals of the smoothing capacitor 10 into a 3-phase AC voltage and supplies variable-voltage and variable-frequency 3-phase AC power to a 3-phase AC load 11 such as an AC motor. An IGBT is generally used as the switching element 2 in most cases and G stands for gate, C collector and E emitter in FIG. 5.
Switching element drive and protective circuits 4 amplify a drive signal from a control arithmetic unit 12 which is an AC load control circuit provided separate from the above power inverter 9 and then turns on or off the switching elements to carry out switching operation required to invert the DC power of the DC power source unit 1 into variable-voltage and variable-frequency 3-phase AC power. A digital signal drive method typified by PWM control is used in most cases. The above drive and protective circuits 4 serve to protect the switching elements 2 based on a sensor signal from an unshown sensor for detecting the overheating or short-circuit current of the switching element 2. Therefore, the control arithmetic unit 12 generally incorporates a microcomputer 5 for carrying out computation for the control of the AC load, receives each phase current of the 3-phase AC load 11 detected by a current detector 13, a 3-phase AC load torque control instruction signal from a car controller 14 and an alarm signal, and controls the power inverter 9 having various protection functions.
Since the car power inverter of the prior art is constituted as described above, the switching element drive and protective circuits 4 must be electrically insulated from the control arithmetic unit 12 for safety or operation principle. A photocoupler 8 composed of an optical semiconductor element is often used as the insulation element.
However, when this photocoupler 8 is used in the extremely severe environment of a temperature cycle for the properties and structure of the optical semiconductor, as in an automobile, the sealing properties of an optical coupling portion deteriorate and coupling (amplification) between a primary side and a secondary side lowers. In the worst case, the primary side and the secondary side are not coupled with each other and there arises such a reliability problem that a signal cannot be transmitted (product life). Since a signal conversion process (electric signal.fwdarw.optical signal.fwdarw.electric signal) is required for insulation, there occur a delay and the rounding of waveform between an input signal and an output signal, and a switching time width required for PWM drive cannot be ensured.